1. Field of the Invention
The present invention relates to the formation of crystals from a supersaturated solution. More specifically, the present invention relates to the formation of substantially pure, or uncontaminated, sodium carbonate decahydrate crystals from solutions supersaturated in sodium bicarbonate and especially solutions obtained in the mining of trona or nahcolite ore deposits.
2. State of the Art
Sodium carbonate, otherwise known as soda ash, is used in the manufacture of glass, chemicals, soaps, detergents, aluminum, and other products. Traditionally, sodium carbonate is produced from trona ore deposits. Such ore deposits are primarily located in California, Utah, and Wyoming. More recently, sodium carbonate is also being produced from nahcolite deposits located primarily in Colorado. Both dry mining and solution mining techniques are used to obtain the necessary ore for sodium carbonate production processes. The methods used in the production of sodium carbonate are continuously evolving as cheaper, faster, and more efficient production methods and processes are discovered or developed.
One of the greatest concerns in soda ash production is the economics of the process. Attempts to decrease soda ash production costs have led to the development of a number of methods for providing a more economical process. The evolution of sodium carbonate production is well documented in U.S. Pat. No. 5,283,054 issued to Copenhafer et al., and U.S. Pat. No. 5,955,043 issued to Neuman et al., which are incorporated herein by reference.
Solution mining is a process for mining trona or nahcolite ore that is gaining popularity. As the readily accessible trona deposits are played-out and long-wall and room and pillar mining techniques become more expensive, solution mining plays a greater role in the production of sodium carbonate from trona ore. Similarly, solution mining of nahcolite ore has become a popular alternative for producing sodium carbonate. Solution mining typically involves the introduction of water, or a dilute solution containing sodium carbonate and possibly sodium bicarbonate, into an ore deposit. A bicarbonate-containing ore is dissolved by the solution creating a liquor containing both sodium carbonate and sodium bicarbonate. The liquor is pumped from a solution-mined ore deposit to a surface plant where it is processed into soda ash or sodium bicarbonate.
Liquors produced by dry mining and solution mining processes typically contain sodium bicarbonate and sodium carbonate, as well as smaller amounts of impurities such as sodium chloride, sodium sulfate, and organics. The presence of sodium bicarbonate concentrations in a liquor may lead to the co-precipitation of unwanted products during crystallization of the liquor. For example, unwanted sodium bicarbonate crystals or sodium sesquicarbonate crystals may co-precipitate with sodium carbonate-containing crystals such as sodium carbonate monohydrate or sodium carbonate decahydrate. It is known, however, that co-precipitation of unwanted crystals may be reduced or eliminated by reducing the sodium bicarbonate concentration of a liquor. One of the challenges facing the soda ash production industry is how to economically reduce the sodium bicarbonate concentration of a liquor such that the sodium bicarbonate concentration will not interfere with the crystallization of substantially pure sodium carbonate-containing crystals.
A number of methods for reducing the sodium bicarbonate concentrations of liquors used in soda ash production processes have been proposed. Typically, the treatments focus upon the conversion of sodium bicarbonate concentrations in the liquor to sodium carbonate concentrations. For example, sodium bicarbonate concentrations may be reduced by adding sodium hydroxide to a liquor that has been evaporated as disclosed by Copenhafer et al. in U.S. Pat. No. 5,283,054. Alternatively, liquor may be subjected to steam stripping to strip CO2 from the liquor and convert sodium bicarbonate to sodium carbonate, as disclosed by Copenhafer et al., U.S. Pat. No. 5,283,054, and by Neuman et al., U.S. Pat. No. 5,955,043.
Heretofore, pre-crystallization treatment of sodium bicarbonate concentrations in liquors served to drive the sodium bicarbonate concentration of the liquor to just at, or below, the sodium bicarbonate equilibrium point of the system. By doing so, the precipitation of sodium bicarbonate or sodium sesquicarbonate crystals during sodium carbonate monohydrate or sodium carbonate decahydrate crystallization can be avoided. For example, FIG. 1 illustrates a phase diagram for a sodium carbonate, sodium bicarbonate, and water system at different temperatures. The solid line connecting points C-D-E-F-G-C defines Region A which represents the region where sodium bicarbonate-containing salts, such as sodium bicarbonate and sodium sesquicarbonate, are not at the equilibrium solid phase at any temperature. Line C–D represents the phase boundary of saturated solutions in equilibrium with sodium carbonate decahydrate and sodium bicarbonate. Line D–E is the phase boundary for saturated solutions in equilibrium with sodium carbonate decahydrate and sodium sesquicarbonate. Liquors used to crystallize sodium carbonate decahydrate are generally treated prior to crystallization to bring the sodium bicarbonate and sodium carbonate concentrations of the liquor within Region A. The broken lines represent liquor compositions that are saturated at the indicated temperatures. Saturated liquor compositions in Region B are normally in equilibrium with sodium bicarbonate, sodium sesquicarbonate, or sodium carbonate monohydrate, at the appropriate temperatures.
The long-standing technical approach towards producing pure decahydrate crystals of sodium carbonate has been to operate well under the equilibrium line to avoid co-crystallization of unwanted species. This has been obtained by treating feed liquors to reduce sodium bicarbonate concentrations therein. Although it has been reported that sodium sulfate has been crystallized under non-equilibrium conditions in 1951, the soda ash industry has not viewed that situation as instructive for soda ash production. See, Hightower, Chemical Engineering, August 1951, p. 104.
Treatment of decahydrate crystallizer feed liquor to reduce the bicarbonate concentration is disadvantageous, however, because the treatment steps are usually expensive and additional process equipment must be added to the overall production process. Therefore, a method for recovering substantially pure, or uncontaminated, sodium carbonate decahydrate, from a liquor having concentrations of both sodium bicarbonate and sodium carbonate is desirable. A method wherein liquor pre-crystallization steps, such as additional caustic or dilution, as well as CO2 stripping, could be reduced or eliminated from the soda ash production process would also be desirable. Furthermore, increasing the amount of carbonate decahydrate recovered from a crystallizer feed liquor, without resorting to additional pretreatment steps to reduce the bicarbonate concentration prior to crystallization, would be of great economic value.